The invention relates to a unit module for a high-frequency antenna for receiving or transmitting a rectilinearly polarized wave. The module has radiating elements in the form of horns and a power supply network assembled from waveguides of rectangular cross-section connected to the horns and also interconnected such that for each horn the total overall length of the supply path is the same.
The invention also relates to a high-frequency antenna comprising such unit modules.
The invention is used, for example, in making planar antennas for receiving television broadcasts transmitted via artificial satellites.
As antenna comprising radiating elements in the form of horns supplied by waveguides is disclosed in Patent Specification DE No. 2641711, which describes a linear antenna module, formed by a row of horns which are manufactured from one glass fibre block with metal-plated surfaces. This row of horns is fed by a main line and also by individual lines connected to the main line. The main line has a rectangular cross-section, is made from aluminium and may be filled with a dielectric material. This main line is realized such that in the plane of the electric field E it constitutes a multi-stage power divider by means of which it is possible to supply at equal powers the waveguides which provide the individual connection of the horns to the main line. Each of these waveguides, of rectangular cross-section, is constituted by a laminated structure having a dielectric material provided between two copper layers, the edges of this structure being metal-plated. The length of the individual supply waveguides and also the point in which they are connected to the main line are chosen such that for each horn the length of the supply path formed by the main line and the individual supply line will be the same. Such a structure has for its object to enable phase differences to be corrected in the power supply to these horns by reducing the length of certain individual power supply lines.
However, such an antenna has several disadvantages. First of all, it has of necessity very high losses since the propagation of the waves in a dielectric medium such as the medium constituted by the laminated structure of the individual power supply lines of the horn is always subjected to high losses, even if the dielectric material is of a very good quality. Using an identical dielectric material in the main line increases the losses still further. Added to that is the fact that the price of a high-grade dielectric material is always very high and considerably increases the cost of the antenna.
Moreover, the antenna module described in the document is of a linear shape, and is fed in series, because of which it is actually very difficult to obtain an accurate in-phase supply of the horns and it is therefore absolutely necessary to effect a length adjustment of the individual supply lines to improve this result. It remains, however difficult to obtain an accurate in-phase supply of all the horns when a wide operating frequency band is required.
In addition, the solution suggested by the document to solve this problem results in a very complicated shape of the antenna and also in assembly and adjusting procedures which are too critical to have them effected during, for example, large series production.
Moreover, to permit the use of this antenna in the reception of television transmission relayed via satellites, the antenna must have special properties.
It should be noted that such an antenna must be capable of receiving a right-hand of left-hand circular polarization, depending on the transmitting satellite.
It is a known fact that the polarization of an electromagnetic wave is defined by the direction of the electric field E in space. If in a point in space the electric field vector E remains parallel to a straight line, which is of necessity perpendicularly to the direction of propagation of the wave, this wave is polarized rectilinearly.
In contrast therewith, the wave is polarized circularly when the end of the electric field vector E describes a circle in the plane perpendicular to the direction of propagation. The polarization is a right-hand circular polarization when E rotates in the clockwise direction, seen in the direction of propagation. In the other case the polarization is a left-hand circular polarization.
A circularly polarized wave may be separated into two linearly polarized waves, which are perpendicular relative to each other and phase-shifted through .+-..pi./2.
The antenna designed for the intended use may consequently be realized in accordance with the following principle: the two perpendicular components, due to the transmission via the satellite of a circularly polarized wave, are pulled-in and thereafter composed with the appropriate phase-shift (.+-..pi./2 depending on the fact whether it is a right-hand or left-hand circularly polarized wave).
Putting this principle into effect assumes the use of a depolarizing radome before the antenna. This radome is designed such that it delays one of the components of the circularly polarized wave, thus producing the necessary phase-shift. The two linearly polarized waves are thus in-phase and the vectorial composition gives a linearly polarized wave which can be received by an antenna having one single linear polarization.
It should moreover be noted that for the intended use, the antenna must satisfy the standards formulated by the CCIR (Comite International de Radiocommunication). These conditions are as follows:
the frequency band must be between 11.7 and 12.5 GHz;
the radiation diagram of the antenna must vary in accordance with a profile according to which an attenuation of 3 dB of the main lobe corresponds to an aperture .theta. of the 2" microwave link, expressed by the relation: EQU .theta..sub.-3 dB =2"
which is the aperture of the microwave link at half-power, and according to which the secondary lobes are attenuated from 30 dB to 12";
the antenna gain G--to--noise temperature T ratio in degrees Kelvin must be: EQU G/T.gtoreq.6 dB..degree.K..sup.-1
Thus, for the indended use, it is important that:
the antenna must be easy to realize and at low cost so as to make the antenna available for the general public,
the antenna must be of a reduced bulk and easy to mount, for example on a roof, so as to ensure that the cost of installation will not increase out of proportion compared with the price of the antenna,
the technical qualities of the antenna must satisfy the standards put forward by the CCIR, and more specifically that the secondary lobes of the network are prevented from occurring.
For that reason the present invention provides a novel high-frequency antenna module which satisfies these conditions.